1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to a method and apparatus for processing a Medium Access Control (MAC) Protocol Data Unit (PDU) in a wireless communication system.
2. Discussion of the Related Art
With reference to FIG. 1, a Universal Mobile Telecommunications System (UMTS) network configuration will be described below.
FIG. 1 illustrates a UMTS network configuration. Referring to FIG. 1, a UMTS system includes a User Equipment (UE), a UMTS Terrestrial Radio Access Network (UTRAN), and a Core Network (CN). The UTRAN includes one or more Radio Network Sub-systems (RNSs) each having a Radio Network Controller (RNC) and one or more Node Bs managed by the RNC. A Node B manages one or more cells.
A radio protocol architecture for the UMTS system will be described with reference to FIG. 2. FIG. 2 illustrates a radio protocol architecture for UMTS. Radio protocol layers are defined in pairs for a UE and a UTRAN, for wireless data transmission. Layer 1 (or L1), the PHYsical (PHY) layer transmits data on a radio link in various wireless transmission techniques. The PHY layer is connected to its higher layer, the MAC layer via transport channels. The transport channels are divided into dedicated transport channels and common transport channels depending on whether they are shared.
The MAC layer, the Radio Link Control (RLC) layer, the Packet Data Convergence Protocol (PDCP) layer, and the Broadcast and Multicast Control (BMC) layer are defined at Layer 2 (or L2). The MAC layer maps logical channels to transport channels and multiplexes a plurality of logical channels onto one transport channel.
The MAC layer is connected to a higher layer, the RLC layer via logical channels. The logical channels are divided into control channels and traffic channels according to the types of information that they carry. The control channels carry control-plane information and the traffic channels carry user-plane information. The control channels include a Common Control Channel (CCCH) carrying common control information, a Dedicated Control Channel (DCCH) carrying control information to a specific UE, a Broadcast Control Channel (BCCH) carrying system information common to a cell, and a Paging Control Channel (PCCH) carrying a paging message. The traffic channels include a Dedicated Traffic Channel (DTCH) carrying user-plane data to a specific UE.
The MAC layer is branched into a MAC-b sublayer, a MAC-d sublayer, a MAC-c/sh sublayer, a MAC-hs/ehs sublayer, and a MAC-e/es or MAC-i/is sublayer depending on the types of specific transport channels that they manage. The MAC-b sublayer manages a Broadcast Channel (BCH) that broadcasts system information, the MAC-c/sh sublayer manages a Forward Access Channel (FACH) that is a common transport channel shared among different UEs, and the MAC-d sublayer manages a Dedicated Channel (DCH) that is a dedicated transport channel for a specific UE. The MAC-hs/ehs sublayer manages a High Speed Downlink Shared Channel (HS-DSCH) that is a transport channel used to transmit high-speed downlink data, and the MAC-e/es or MAC-i/is sublayer manages an Enhanced Dedicated Channel (E-DCH) that is a transport channel used to transmit high-speed uplink data.
The RLC layer ensures the Quality of Service (QoS) of Radio Bearers (RBs) and is responsible for data transmission. The RLC layer has one or two independent RLC entities for each RB in order to ensure QoS. To support various QoS levels, the RLC layer provides three RLC modes, Transparent Mode (TM), Unacknowledged Mode (UM), and Acknowledged Mode (AM). In addition, the RLC layer controls a data size to suit radio data transmission at a lower layer. For controlling a data size, the RLC layer segments or concatenates data received from a higher layer.
The PDCP layer is located above the RLC layer. The PDCP layer enables efficient data transmission in IP packets such as IP version 4 (IPv4) or IP version 6 (IPv6) packets on a radio link having a relatively narrow bandwidth. For this purpose, the PDCP layer performs header compression. Since only necessary information is transmitted in the header of data header through header compression, the transmission efficiency of the radio link is increased. The PDCP layer exists mainly in a Packet Switched (PS) domain because header compression is it basic function. To provide an efficient header compression function for each PS service, one PDCP entity is defined for each RB. However, if the PDCP layer exists in a Circuit Switched (CS) domain, the PDCP layer does not provide the header compression function.
The BMC layer is also above the RLC layer, for scheduling a cell broadcast message and broadcasting the cell broadcast message to UEs within a specific cell.
The Radio Resource Control (RRC) layer, which is located at the lowest part of Layer 3 (or L3), is defined only on the control plane. The RRC layer is involved in establishing, reestablishing, and releasing RBs, controls L1 or L2 parameters, and controls logical channels, transport channels and physical channels. An RB refers to a logical path formed at L1 and L2 in the protocol stack, for data transmission between a UE and a UTRAN. In general, setup of an RB is the process of specifying radio protocol layers and channels necessary to provide a specific service and setting specific parameters and an operation scheme.
With reference to FIGS. 3 and 4, a conventional MAC PDU transmission method will be described. FIG. 3 illustrates an exemplary conventional MAC PDU transmission method.
Conventionally, if a MAC PDU or a segment of a MAC PDU is too large for a Transport Block (TB), a MAC segmentation entity of a transmitter segments the MAC PDU or the MAC PDU segment and constructs a TB with a segment, while storing the other segments. A Hybrid Automatic Repeat reQuest (HARQ) entity of the transmitter transmits the TB to a receiver. If the HARQ entity fails to receive an ACKnowledgement (ACK) from the receiver after transmitting the TB a maximum retransmission number of times, the HARQ entity discards the TB buffered in an HARQ process buffer. However, the remainder of the MAC PDU corresponding to the failed TB may be retained in the MAC segmentation entity and thus the transmitter may transmit the remainder of the MAC PDU to the receiver.
Even though the receiver successfully receives the remainder of the MAC PDU, it cannot assemble the whole MAC PDU because of the failed MAC PDU segment. Accordingly, the receiver discards the successfully received part of the MAC PDU. This means that radio resources are dissipated due to unnecessary data transmission.
Referring to FIG. 3, in the case where a first segment of a MAC PDU is failed in HARQ transmission, a second segment of the MAC PDU buffered in an HARQ process buffer may be transmitted. Even though the receiver receives the second segment successfully, the receiver cannot construct the whole MAC PDU because of the failed first segment. Therefore, the receiver discards even the successfully received second segment.
FIG. 4 illustrates another exemplary conventional MAC PDU transmission method.
Conventionally, when receiving an RLC Service Data Unit (SDU) from a higher layer, a UE expects successful transmission of the RLC SDU within a maximum delay time. Therefore, upon receipt of RLC SDUs, the UE activates a discard timer Timer_Discard, constructs an RLC PDU with the received RLC SDUs, and expects the RLC PDU to be transmitted until before expiration of the discard timer Timer_Discard. Hence, upon expiration of the discard timer Timer_Discard, the UE clears the RLC SDU from a buffer.
Because the discard timer Timer_Discard is set to a maximum delay time allowed for the RLC SDU, a network discards the RLC SDU even though the network receives the RLC SDU after the maximum delay time. Therefore, the UE also discards the RLC SDU upon expiration of the discard timer Timer_Discard. The reason for discarding the RLC SDU is to prevent the overflow of the RLC buffer and the time delay of new data to be transmitted that might be caused by the data received after a time delay and thus not processed.
Even though the deletion of the RLC SDU from the RLC SDU buffer upon expiration of the discard timer Timer_Discard, there may remain MAC PDU segments yet to be transmitted in a MAC-i/is segmentation entity of the MAC layer in the UE. Upon receipt of a new RLC SDU, the UE transmits an RLC PDU constructed with the new RLC SDU after transmitting the remaining MAC PDU segments. That is, although the UE wants to transmit a second RLC SDU after discarding a first RLC SDU, the second RLC SDU may have an additional time delay due to the transmission of the MAC PDU segments yet to be transmitted in the MAC layer.
As described above, the conventional MAC PDU transmission methods may dissipate radio resources and cause a time delay during transmission because a UE transmits an unnecessary MAC PDU segment.